Slug-like creatures that evolved at unnatural speed from microbes carried on Spock's torpedo casing after it came to rest on the unstable Genesis planet in 2285.
The worms of the Genesis planet began as microscopic organisms on the surface of the photon torpedo tube used as Spock's coffin. Caught in the Genesis effect, they grew and evolved within days into slimy, slug-like animals and then into much larger, eel-like forms.
They appear in Star Trek III: The Search for Spock. David Marcus and Saavik, surveying the new world from the USS Grissom, detect an unexpected life sign, beam down and find the small worms crawling over the empty casing, which is their first clue that the planet's evolution is running out of control.
Later, Kruge's landing party finds the same site. The worms have grown considerably, and one coils around the Klingon commander, who throttles it with his bare hands. They presumably perished when the planet tore itself apart.